Abstract: The perthite megacrysts from charnockitic and non-charnockitic members of a sialic intrusive complex in the Varberg area of south-west Sweden have been investigated by powder X-ray diffraction. The extent of Al/Si order in the K-feldspar host lattices of the megacrysts is restricted in all but the major alkali granite differentiate. A direct relationship between the hydration state of the containing rock and the degree of order of the megacryst lattices is indicated. High lattice strain is common to all the megacrysts but decreases progressively with increasing triclinicity.